The present invention relates to a multi-voltage vehicle electrical system for a motor vehicle, including at least one multi-voltage generator, a first energy store, whose energy may be fed into a first low-voltage vehicle electrical system branch having low-voltage consumers, and a second energy store, whose energy may be fed into a second high-voltage vehicle electrical system branch having high-voltage consumers, and at least one controllable switching device, using which the energy generated by the multi-voltage generator may be distributed to the first and second vehicle electrical system branches.
A multi-voltage vehicle electrical system for a motor vehicle of this type is known from German patent document DE 103 30 703 A1. Multi-voltage vehicle electrical systems, as are used in manifold forms in modern motor vehicles, typically include two vehicle electrical system branches of different rated voltages. Consumers of lower power consumption are connected to the low-voltage vehicle electrical system branch, whose rated voltage may be 12 V, for example. High-power consumers, such as heaters, electrical power steering, electric-motor brakes, etc., are connected to the high-voltage vehicle electrical system branch, whose rated voltage may be 24 V, for example. Each vehicle electrical system branch has an energy store assigned thereto. The use of batteries as energy stores in both vehicle electrical system branches and also the use of a battery as the energy store of the low-voltage vehicle electrical system branch and a capacitor circuit of high capacitance as the energy store of the high-voltage vehicle electrical system branch are known. Both vehicle electrical system branches are typically fed by a multi-voltage generator, which may be implemented as an integrated starting generator, for example. The term “multi-voltage generator” is understood in the scope of the present application as any generator whose output voltage level varies or is variable chronologically.
In the known multi-voltage vehicle electrical systems, and also in the multi-voltage vehicle electrical system disclosed in above-mentioned DE 103 30 703 A1, the rectified generator output voltage is fed directly into the high-voltage vehicle electrical system branch. This is connected via a DC/DC converter to the low-voltage vehicle electrical system branch. The DC/DC converter not only provides a transformer device for adapting the various voltage levels in the individual vehicle electrical system branches, but rather also allows, through suitable activation of its switchable elements, a distribution of the energy generated by the generator to the two vehicle electrical system branches as a function of operating parameters. For example, the charge state of the energy stores and/or the instantaneous load of the individual vehicle electrical system branches by connected consumers come into consideration as relevant operating parameters. A comparable construction is shown by the vehicle electrical system disclosed in European patent EP 136 00 90 B1, which is additionally also suitable for recuperation operation.
The known vehicle electrical system topologies have the disadvantage of the requirement for the DC/DC converter as a transformer and switching element. Specifically, converters of this type are comparatively costly and complicated in construction and activation. In addition, they represent a significant performance limitation, which reduces efficiency very strongly in particular in recuperation operation, in which very large amounts of power are fed into the vehicle electrical system in a short time by reclaiming energy during braking procedures.
It is therefore an object of the present invention to provide a novel, simplified topology for a multi-voltage vehicle electrical system for a motor vehicle, which manages without a DC/DC converter and nonetheless has a high compatibility to existing low-voltage vehicle electrical system topologies.
It is a further object of the present invention to provide an operating method for an improved vehicle electrical system of this type.
The first object is achieved in connection with a multi-voltage-vehicle electrical system in which the first and the second energy stores are connected in series to one another and the first vehicle electrical system branch is connected on one hand to a node between the first and the second energy store and on the other hand to a reference potential of the first energy store, while the second vehicle electrical system branch is connected in parallel to the series circuit made of first and second energy stores. The controllable switching device is switchable between a first switching configuration, in which the multi-voltage generator is in parallel to the first energy store and to the first vehicle electrical system branch, and a second configuration, in which the multi-voltage generator is in parallel to the series circuit made of first and second energy stores and to the second vehicle electrical system branch.
In contrast to the prior art, the vehicle electrical system according to the invention couples its two branches directly, in that the energy stores are connected to one another in series. It is also possible through the use of a suitable switching device to keep both vehicle electrical system branches decoupled in the “view” of their consumers, so that known and proven vehicle electrical system branch topologies, in particular proven low-voltage vehicle electrical system topologies, may be adopted.
The low-voltage vehicle electrical system branch is thus connected at a node between the two energy stores and is therefore connected in parallel to the first energy store, with which it has a shared reference potential. The second energy store, in contrast, takes the upper potential of the first energy store as the reference potential, i.e., starts at the voltage level of the low-voltage vehicle electrical system branch. The high-voltage consumers of the high-voltage vehicle electrical system branch are in parallel to the series circuit made of first and second energy stores and therefore “see” the sum of the voltage level of both energy stores.
Both energy stores and thus both vehicle electrical system branches are fed by the same generator via suitable switching device. The switching device is designed in such a manner that the generator output voltage is applied either only via the first energy store or via the series circuit of both energy stores, care having to be taken in the first case that no current flows from the second energy store to the first energy store. In addition to the desired removal of the DC/DC converter, the vehicle electrical system topology according to the invention has the advantage that the second energy store must only have a comparatively low-voltage level. While its maximum voltage level in vehicle electrical systems according to the prior art must essentially correspond to the rated voltage of the high-voltage vehicle electrical system branch, it is sufficient in the vehicle electrical system according to the invention if the maximum voltage level of the second energy store approximately corresponds to the difference of the rated voltages of the two vehicle electrical system branches.
The controllable switching device may include a first switchable interrupter between the connection nodes of the first vehicle electrical system branch and the terminal of the multi-voltage generator facing away from its reference potential. This corresponds to a switch in the direct connection line between the output of the generator and the pole of the first energy store, which is not at ground and is typically positive in vehicle electrical systems.
In a first, especially simple embodiment of the invention, the controllable switching device also includes a diode, which is connected between the terminals of the multi-voltage generator and the second energy store facing away from their particular reference potentials in such a manner that when the first interrupter is closed, no current may flow from the second energy store to the first energy store. In other words, this means that the output of the generator is also connected to the positive pole of the second energy store, via a diode which is situated after the branch to the first interrupter. The diode is oriented in such a manner that when the first interrupter is opened, the generator output voltage is applied via the series circuit made of first and second energy stores, i.e., both energy stores are charged, while if the first interrupter is closed, only the first energy store is charged and a current flow from the second energy store to the first energy store would occur in the blocking direction of the diode and is thus suppressed.
In an alternative embodiment of the invention, instead of the diode, a second switchable interrupter may be provided, which is connected between the terminals of the multi-voltage generator and the second energy store facing away from their particular reference potentials in such a manner that when the first interrupter is closed, a current flow from the second energy store to the first energy store may be prevented by opening the second interrupter. It is obvious that in this embodiment, the two interrupters must be activated suitably to suppress the desired current flow. This disadvantage of increased complexity of the activation is compensated for, however, by the advantage that the voltage drop via the diode is dispensed with.
A comparatively “large” battery is suitable as the first energy store, which must typically bear the main load of supplying the consumers. A—preferably “smaller”—battery may also be used for the second energy store; alternatively, however, a capacitor circuit made of one or more capacitors of high capacitance, so-called super capacitors or supercaps, may also be used.
The above-mentioned second object is achieved by a method according to the present invention.
For efficient and cost-effective operation of a vehicle electrical system of this type, it is favorable to switch the controllable switching unit essentially no-load. This means that in both above-mentioned embodiments, i.e., both in the embodiment having diode and also having second interrupter (which are jointly included by the term “at least unidirectional disconnection unit”), in accordance with the operating method according to the invention, before the first interrupter is closed, the generator output voltage is equalized to the potential of the terminal of the first energy store facing away from its reference potential. In other words, this means that if only the first energy store is to be charged, i.e., typically the output of the generator is to be connected to its positive pole, first the current potential at this pole is detected and the output voltage of the generator is equalized to this potential. Correspondingly, closing the first interrupter does not result in a current flow, i.e., it is switched in a no-load manner. Subsequently, the output voltage of the generator may be moved to a desired charge level. Appropriate sensors may be provided for detecting the potentials of the energy stores.
In the previously explained embodiment having two interrupters, the second interrupter is opened before the equalization of the generator output voltage in a favorable refinement of the method according to the invention. This prevents current from flowing from the second energy store to the first energy store upon closing of the second interrupter.
If the second energy store is also to be charged, in the previously explained embodiment having a diode, only the first interrupter is to be opened, preferably after equalization of the generator output voltage to the potential of the positive pole of the first energy store. In the second embodiment having two interrupters, in contrast, the second interrupter must additionally be closed. In a favorable embodiment of the method according to the invention, before the second interrupter is closed, the first interrupter is first opened and the generator output voltage is equalized to the potential at the terminal of the second energy store facing away from its reference potential. This measure is also used for no-load switching of the second interrupter, similarly to the above explanations.
The fundamental strategy of the activation of the switching device is to open the first interrupter as rarely as possible, so that the low-voltage vehicle electrical system branch essentially “sees” a typical low-voltage energy store. In a refinement of the operating method according to the invention, the first interrupter is thus essentially only opened when the charge state of the second energy store has fallen below a predetermined charge level. In addition, opening the first interrupter may also be done in recuperation operation.
As already noted, any generator having varying or variable output voltage fundamentally comes into consideration as the multi-voltage generator. Thus, for example, it is fundamentally sufficient to use a generator whose rectified output voltage has a high ripple, the controller of the switching device being adapted to the frequency of the ripple, so that in “wave valleys” only the first energy store and in “wave peaks” the series circuit made of first and second energy stores together is charged. However, using a dual-voltage generator is more favorable, which is activated together with the switching device as a function of operating parameters, i.e., in particular as a function of the charge state of the second energy store. The use of a generator having arbitrarily activatable output voltage is still more favorable, with the aid of which a capacitor circuit used as the second energy store may be charged using linear charge ramps, for example, in the scope of an especially favorable embodiment of the operating method according to the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.